Nonlinear, time-varying loads such as, for instance, three-phase arc furnaces can give rise to considerable disturbances that affect the power supply system, in particular what termed flicker. For this reason power supply installations for nonlinear loads varying with time generally include a reactive power compensator. The reactive power compensator is connected in parallel with the nonlinear, time-varying load. In the simplest case the reactive power compensator can be embodied as what is termed a TCR (thyristor-controlled reactor), see EP 0 847 612 B1, for example.
A power supply device of the kind cited in the introduction is known for example from DE 10 2008 049 610 A1 and EP 2 202 863 A1.
While it is expressly mentioned in the prior art that the multilevel converter can be connected to the phases of the power system alternatively in a star or delta configuration, the multilevel converter is, though, in practice always connected to the phases of the power system in a delta configuration. The reason for this is that the multilevel converter must be able at a certain instant to absorb energy via one of its strands and at the same time deliver energy via another of its strands. This—in the case both of a delta and of a star connection—can very quickly cause permissible voltage limits to be exceeded, resulting in blocking of the multilevel converter.
When the multilevel converter is connected in a delta configuration it is, however, known to maintain the voltage limits by generating within the circuit formed by the strands of the multilevel converter a corresponding current flow by means of which developing voltages will be kept within the permissible range. A zero sequence current flow of such kind is not readily possible in the case of a star connection.
It is furthermore known to provide within the multilevel converter an alternating-current (AC) voltage circuit which is operated on a high-frequency basis and which connects the individual direct-current (DC) voltage circuits of all the modules of all the strands of the multilevel converter to one another. In this embodiment the exchange of energy takes place by way of a DC-to-AC voltage conversion alongside downstream AC-to-DC voltage conversion. This embodiment of the multilevel converter can be used both in a delta connection and in a star connection. In this case, however, the embodiment of the multilevel converter is very complex. Moreover, the possible energy transfer per unit of time is relatively low.